setup.py: Require ordereddict for Python 2.6

Without this, a client who uses the alphabetical attribute sorting feature of `HTMLSerializer` on Python 2.6 will get:

```python
  File "/Users/marca/dev/git-repos/bleach/.tox/py26/lib/python2.6/site-packages/html5lib/serializer/htmlserializer.py", line 309, in render
    return "".join(list(self.serialize(treewalker)))
  File "/Users/marca/dev/git-repos/bleach/.tox/py26/lib/python2.6/site-packages/html5lib/serializer/htmlserializer.py", line 196, in serialize
    from ..filters.alphabeticalattributes import Filter
  File "/Users/marca/dev/git-repos/bleach/.tox/py26/lib/python2.6/site-packages/html5lib/filters/alphabeticalattributes.py", line 8, in <module>
    from ordereddict import OrderedDict
ImportError: No module named ordereddict
```